The "signal path" is a concept which, as normally used, is meaningless. I can see at least 8 components which could affect the signal, quite apart from the power supply.

This is true!

I am considering only the main components: tube, interstage transf. and mosfet.

If I compare this project to others this is a not common design, also my PF2007 system is more complex because need the output capacitor.PF2007: the system

I see as large influence in the sound the cathode capacitor that I cannot eliminate but it can be considered an advantage because you can use it to modify the sound.

Some candidates are:
1) Sanyo OS-Con - very neutral but a little bit too hard
2) ELNA Cerafine - very smooth, good to lay normal CD
3) ELNA Silmic II - less smooth of Cercafine but still too much

In my last GM70 SE dc coupled GM70 amplifier with Fiat transformers
I have got a good benefit using a led ( in the optocoupled) instead of cap/res. on cathode but here I cannot apply it because the low frequecny band will be too much compromised.